1. Field of the Invention
The present invention relates to a light pipe, and more particularly, to a hollow tapering light pipe.
2. Description of the Prior Art
With the progress of optics, projectors have found broad use in many applications. Generally speaking, projected light intensity is usually an important factor while designing a projector, as the projection quality improves with greater intensities of the projected light.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a prior art light pipe 10 for a projector system 12. The projector system 12 comprises a light source 14 to generate light 11 and to guide the light 11 to the light pipe 10, and an image device 16 to project the light gathered by the light pipe 10 to a screen 13 so as to form an image. The light pipe 10 is a hollow rectangular pipe set between the light source 14 and the image device 16 to gather and guide the light 11. The light pipe 10 comprises an entrance pupil 18 and an exit pupil 19. The entrance pupil 18 is used to gather the light 11 generated by the light source 14. Then the light 11 is guided to the image device 16 through the exit pupil 19.
Please refer to FIG. 2. FIG. 2 is a position distribution of the light incident upon the entrance pupil 18 in FIG. 1. The transverse axis is a distance in the plane 15 in FIG. 1 from the center of the entrance pupil 18. The longitudinal axis is the light intensity. As shown in FIG. 2, the light closer to the center of the entrance pupil 18 is stronger, and the light farther from the center of the entrance pupil 18 is weaker. In addition, the light pipe 10 does not gather all of the light 11. The light 24 closer to the center of the entrance pupil 18 enters the light pipe while the light 25 farther from the center of the entrance pupil 18 fails to be gathered by the light pipe 10.
Please refer to FIG. 3. FIG. 3 is the incident angle distribution of the light incident upon the entrance pupil 18. The transverse axis of FIG. 3 is the incident angle of the light on the plane 15, and the longitudinal axis is the light intensity. As shown in FIG. 3, the incident angle of a traditional light bulb is in the range of 18-20 degrees. Because the light pipe 10 is a rectangular pipe with homogeneous cross-sections, the angle of the light leaving of the exit pupil 19 is also in the range of 18-20 degrees. Usually, the optic device for the image device 16, such as the liquid crystal display (LCD) or the digital mirror device (DMD), has an incident angle limitation. Owing to the incident angle limitation, the angle of the light leaving the exit pupil 19 cannot be too large. Usually, light with an angle over 30 degrees cannot be modulated by the liquid crystal display.
Please refer to FIG. 4. FIG. 4 is a schematic diagram of another prior art light pipe 20 for the projector system 12 in FIG. 1. The light pipe 20, differing from the light pipe 10, has a tapered body. The exit pupil 23 of the light pipe 20 is larger than the entrance pupil 22 to reduce the incident angle of the light.
Generally speaking, the light pipes 10, 20 are designed to reduce the light 25 leaking from the light pipes 10, 20 while maintaining a small light angle. Thus, the light gathering efficiency is improved, and the light intensity is increased. However, owing to the fact that the prior art light pipes 10, 20 have the entrance pupil 18, 22 smaller than or equal to the exit pupil 19, 23, the light 25 leaking from the light pipe 10, 20 is usually quite strong without increasing the volumes of the light pipes 10, 20. Though an arc lamp with a longer arc length can be used as the light source to reduce the incident angle in FIG. 3, it also makes the position distribution of the light in FIG. 2 smoother. Then the light leaking from the light pipe 10, 20 increases, degrading the light gathering efficiency.
It is therefore an object of the present invention to provide a light pipe having an exit pupil with a shape that is scaled from the shape of the entrance pupil to increase the light gathering efficiency of the light pipe, and to improve over the disadvantages of the prior art.
The light pipe is designed for a projector system. The projector system comprises a light source to generate light and to guide the light to the light pipe, and an image device to produce an image using the light gathered by the light pipe. The light pipe is a tapered hollow tube set between the light source and the image device. The light pipe comprises an entrance pupil and an exit pupil. The entrance pupil is a square opening at one end of the light pipe. The light generated by the light source is guided to the light pipe through the entrance pupil. The exit pupil is set at another end of the light pipe, and the area of the exit pupil is smaller than the area of the entrance pupil. The light gathered by the light pipe is guided to the image device through the exit pupil. The shape of the exit pupil is scaled from the shape of the entrance pupil so as to increase the light gathering efficiency of the light pipe.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.